Effect of Thermo-mechanical Treatment on High Temperature Tensile Properties and Ductile–Brittle Transition Behavior of Modified 9Cr-1Mo Steel

In the present work, a modified 9Cr-1Mo steel is subjected to normalizing and tempering treatment with or without an intermediate rolling, which was carried out at 1050 °C in austenitic phase and 550 °C in metastable austenitic phase. The tempering was carried out at 700 °C and 750 °C to improve the strength by precipitation hardening and refining the microstructure. The ductile to brittle-transition temperature (DBTT) and tensile properties have been evaluated at 20 °C, 550 °C and 650 °C for the material subjected to various thermomechanical treatments. Rolling performed in austenitic phase showed improvement in the upper shelf energy and reduction in the DBTT compared with the material rolled in metastable austenitic phase. Rolling in metastable austenitic phase improved the yield strength at elevated temperature (39 pct at 20 °C, 31 pct at 550 °C and 91 pct at 650 °C) and was accompanied by a reduction in ductility (21 pct at 20 °C, 32 pct at 550 °C and 35 pct at 650 °C) compared with the as-received condition. The influence of grain size, low angle boundaries and precipitates on DBTT and high temperature strength has been studied using EBSD and TEM analyses. An increase in high temperature strength is found to increase with the area fraction of fine M23C6/MX precipitates, whereas low angle boundaries and effective grain size influenced the DBTT behavior. The combined effect of precipitation strengthening and strain hardening leads to improvement in the high temperature mechanical strength while maintaining adequate toughness. The effect of aging (for 72 hours at 650 °C) on heat-treated and -rolled samples showed a small decrease in yield strength at 20 °C. A decreasing trend of fracture toughness is observed with the increase in yield strength.